Solar cells are photovoltaic cells or modules, which convert sunlight directly into electricity. Photovoltaic (PV) cells are made of semiconductors, most commonly silicon. When light strikes the cell, a certain portion of it is absorbed within the semiconductor material, such that the energy of the absorbed light is transferred to the semiconductor and an electrical current is produced. By placing metal contacts on the top and bottom of the PV cell, the current can be drawn off to use externally. The current, together with the cell's voltage, defines the wattage that the solar cell can produce.
Silicon, especially in its crystalline form, is a common material used for producing solar cells. Most solar cells are made from crystalline silicon, doped with boron and phosphorus to produce a p-type/n-type junction. Polycrystalline silicon can be used in solar cell fabrication to cut manufacturing costs, although the resulting cells may not be as efficient as single crystal silicon cells. Amorphous silicon, which has no crystalline structure, may also used, again in an attempt to reduce production costs. Other materials used in solar cell fabricated include gallium arsenide, copper indium diselenide and cadmium telluride.
A typical arrangement of a silicon solar cell is as follows:
(a) a back contact;
(b) a P-type Si;
(c) an N-type Si;
(d) an antireflective coating;
(e) a contact grid; and
(f) a cover glass.
Because silicon is extremely reflective, an antireflective coating is typically applied to the top of the cell to reduce reflection losses. A glass cover plate is typically applied over the antireflective layer to protect the cell from the elements.
Low and medium efficiency solar cells are preferably produced in an efficient manner in order to keep the overall costs as low as possible. As such, these solar cells may be manufactured in a continuous, high throughput line in which handling of the wafers is kept to a minimum. The number of process steps is kept as low as possible and the process steps are selected so as to allow continuous processing with no or minimal interruption.
Conventional solar cells can be made using crystalline silicon wafers. The Si (+4) wafer starts as a p-type with a boron (+3) dopant. To better capture light, the wafer may be texturized with hydroxide or nitric/hydrofluoric acids so that light is obliquely reflected into the silicon. The p-n junction is formed by diffusion with phosphorus using vapor deposition and a surface passivation layer is applied, again in vacuum equipment, to impart the silicon nitride film.
In a standard process of silicon solar cell fabrication, the front side of the silicon wafer is coated with an anti-reflective passivation layer, which is typically comprises silicon nitride. This silicon nitride layer serves the dual purpose of maximizing the percentage of light absorbed by the cell (not reflected), as well as passivating the surface, which prevents electron recombination at the surface and thus increases cell efficiency.
After anti-reflective coating deposition, the cell is typically patterned with a frit-containing silver paste, using a screen printing method. The silver paste is then fired in order to penetrate the nitride passivation layer and form an electrically conductive contact with the bulk silicon material. At the same time, the circuit can be completed on the backside of the cell, for example with aluminum and silver pastes, silver to make contact with silicon and aluminum to form a back surface field.
As described for example in U.S. Pat. No. 5,698,451, the subject matter of which is herein incorporated by reference in its entirety, a typical method for forming a silicon solar cell involves the following steps: (1) providing a silicon substrate which has a p-n junction and a layer of silicon nitride on its front surface (adjacent the junction), (2) selectively coating the silicon nitride layer with a paste or ink that contains silver particles and a glass frit, so that the coating forms a selected contact pattern on the silicon nitride, and (3) heating the substrate to a temperature in excess of about 750° C., for a time sufficient to rapidly cause the silver/glass frit coating to penetrate the silicon nitride layer and form an ohmic contact on the front surface of the substrate.
The '451 patent also discloses a process involving the following steps: (1) providing a silicon substrate which has a p-n junction and a layer of silicon nitride on the front surface, (2) coating the back side of the silicon substrate with an aluminum paste, (3) heating the silicon substrate to rapidly and efficiently cause the aluminum to form an adherent conductive back side contact, (4) coating the silicon nitride with a paste containing silver particles and glass frit so as to form a grid-shaped electrode pattern on the silicon nitride, and (5) heating the substrate to a temperature in excess of 760° C., for a period of time sufficient to cause the metal and frit components in the paste to penetrate the silicon nitride layer and form an adherent conductive front side contact.
There are several notable disadvantages to the current method of patterning silicon solar cells including (1) breakage due to contact involved with screen printing; (2) loss of cell efficiency due to shading of the front side of the cell because of the grid; and (3) loss of cell efficiency due to improper electrical contact between the silver paste and the underlying silicon due to incomplete nitride dissolution and other contaminants present at the silver/silicon interface.
Therefore, improvements to the current screen printing method for patterning solar cells are desirable.
Conductor formation techniques as an alternative to screen printing paste have also been suggested, including for example deposition of inkjet resist and etch of the anti-reflective coating (ARC), aerosol deposition of silver paste, laser ablation of ARC and photolithography. Thereafter, electroless and/or electrolytic plating can be used to build the conductor. For example, a thin layer of electroless nickel can be used to make electrical contact to the silicon which can then be plated with copper—the thin layer of nickel is generally necessary to prevent the copper from poisoning the silicon.
However, further improvements are still needed with these alternative conductor formation techniques.